fix type of function bodies in Typed_ast
fix type of function bodies in Typed_ast

Summary:
`BodyExpression` is for arrow functions, but was being used in `Typed_ast` for non-arrow cases. Switched to using `BodyBlock` instead.

This is part of a stack to make the AST stricter so that expressions can't be used as bodies of non-arrows by construction.

Reviewed By: panagosg7

Differential Revision: D12867519

fbshipit-source-id: 1f1c39df5c48dd31e14148f05d3b7aa7da3f38e9

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow